1. Field of the Invention
The present invention relates to a high-frequency amplifier for amplifying a multicarrier signal generated by combining plural carrier waves modulated independently.
2. Description of the Related Art
CDMA (Code Division Multiple Access) system inherently has confidentiality and interference-resistibility and is the multiple access system capable of making efficient use of radio frequencies. Therefore, CDMA system is applied to various communication systems.
Furthermore, CDMA system is being actively applied to mobile communication system since the transmitting power control technology with high response and high accuracy has been established so that the near-far problem is solvable in recent years.
The transmitting section of a radio base station of a mobile communication system to which CDMA system described above is applied is composed of high-frequency amplifiers 101-1 to 101-N which amplify the power of plural N of RF signals that have different frequencies of carrier waves respectively and whose frequencies are assigned based on a given frequency allocation and on the zone configuration and a combiner 102 which generates multicarrier signals to be fed to an antenna system by combining RF signals given respectively by the output of high-frequency amplifiers 101-1 to 101 N as shown in FIG. 13.
It is now assumed that the number N of the RF signals described above is 2, for simplicity. The frequencies of the carrier waves of these RF signals are indicated by f1 and f2, respectively.
In the transmitting section configured as described above, the high-frequency amplifiers 101-1 and 101-2 amplify the electric powers of first and second RF signals, respectively, which contain the frequencies f1 and f2, in the occupied bands.
Since these high-frequency amplifiers 101-1 and 101-2 amplify the first and second RF signals, respectively, even if nonlinear regions are contained in the characteristics of amplification elements provided in the high-frequency amplifiers 101-1 and 101-2, noises are not generated due to cross modulation (intermodulation) of these RF signals. Furthermore, such noises will be hereinafter referred to simply as cross modulation distortion.
In the prior art described above, as the number N of the frequencies of carrier waves assigned to the radio base station increases, the number N of the amplifiers 101-1 to 101-N increases, thus increasing the size of the hardware.
As for the configuration of the hardware of the radio base station, it is generally required to be adaptable to the maximum value Nmax of the number of carrier waves that can be assigned.
However, as for the radio base station, as the size of the hardware increases, constraints on the office establishment, such as floor space and volume needed for installation and power consumption, may become more stringent and the reliability may deteriorate.
The increases of constraints and deterioration of the reliability may be alleviated by a combiniation of a combiner for combining the plural N of RF signals and a single common amplifier for amplifying the multicarrier signals gained from the output of the combiner. However, the common amplifier is required to have high linearity enough to suppress the level of the cross modulation distortion of the plural N of RF signals below a desired upper level.
Furthermore, the dynamic range of the common amplifier is required to become wider with increasing the number N of the RF signals described above and with increasing the area of the wireless zone formed by the radio base station.
Accordingly, even though the common amplifier is technically realizable, it is rarely put into practical use because of costs and other constraints.
The cross modulation distortion described above, as shown in ((4) in FIG. 14), is generally generated as a component of frequencies of summation and subtraction between the frequencies f1, to fN ((1) and (2) in FIG. 14) of plural N of carrier waves assigned to the radio base station and the cross modulation distortion ((3) in FIG. 13) of frequencies equal to the frequency difference xcex94f among the frequencies f1 to fN on the frequency axis, as for that distortion, it is referred to hereinafter as the basic modulation product.
For simplicity, the frequency difference xcex94f is assumed hereinafter that it is the frequency difference between the frequencies of adjacent carrier waves as given by
xcex94f=fk+1xe2x88x92fk
where k is an arbitrary integer (k=1 to (Nxe2x88x921)).
However, the impedance or inductance of a line to be grounded inside the common amplifier generally increases with increasing the frequency xcex94f of the basic modulation product described above. Similarly, the level of the basic modulation product increases.
That is, as the frequency difference xcex94f between the frequencies of the plural N of RF signals contained in the multicarrier signals to be amplified increases, the level of the generated cross modulation distortion increases.
Accordingly, it has been necessary that the conventional common amplifier described above be made of a circuit having an impedance that is low enough to tolerate the level of the cross modulation distortion.
In a mobile communication system to which wideband CDMA system is applied, the frequency xcex94f of the basic modulation product generally assumes large values of more than 10 MHz to tens of MHz.
Therefore, it has been difficult to use a common amplifier including the circuit of low impedance as described above unless the following conditions hold:
(1) Increase of the power consumption is tolerable under other constraints including running costs.
(2) It is possible to cope with the technical constraints on mechanical dimensions and thermal design of amplifier elements and other elements.
It is an object of the present invention to provide a high-frequency amplifier which includes small-scale hardware but is capable of flexibly adapting itself to various frequency allocations.
It is another object of the invention to provide a high-frequency amplifier including a small-scale circuit that can accomplish high power efficiency and a high SN ratio.
It is a further object of the invention to provide a high-frequency amplifier that can stably maintain SN ratio even if there are a large number of carrier waves used for the generation of multicarrier signals to be amplified or even if the frequencies of these carrier waves are allocated in a various or variable manner.
It is yet another object of the invention to provide a high-frequency amplifier applied in electronic appliances, equipments, or systems, which can reduce them in price and size, and improve them in reliability. It is also to provide the high-frequency amplifier applied in electronic appliances and such which can also allow the maintenance and operation to be more efficient and be done at reduced costs.
The above objects are achieved by a high-frequency amplifier comprising an amplification means for amplifying a multicarrier signal generated by combining plural carrier waves modulated independently and a filtering means connected to the output terminal of the amplification means, having a passband lying within the range of the band occupied by the multicarrier signal,such transfer characteristics as to suppress the level of noise below a predetermined upper limit, the noise being generated as a modulation product between the multicarrier signal and modulation product having a frequency equal to the frequency difference xcex94f in the frequency axis as a resulting product between said carrier waves, and a rejection band including the frequency difference xcex94f in its range.
In this high-frequency amplifier, the noise described above mainly includes nonlinear distortion generated due to the nonlinearity of the amplification means.
Accordingly, as long as filtering characteristics adaptive to the characteristics of the amplification means and desired frequency allocation are established into the filtering means beforehand, the small-scale circuit including the amplification means and the filtering means described above can accomplish high power efficiency and high SN ratio in this high-frequency amplifier.
The aforementioned object is also achieved by the filtering means having a rejection band which has a band lower than or equal to the frequency equal to said frequency difference xcex94f allotted to it.
In this high-frequency amplifier, the SN ratio is stably maintained without changing the character of the filtering means even if there are a large number of carrier waves used for the generation of multicarrier signals to be amplified or even if the frequencies of these carrier waves are allocated in a various or variable manner.
The above-described objects are also achieved by the filtering means having a rejection band having a band in which second harmonic of said multicarrier signal is distributed or a higher rejection band containing the said band allotted to it.
Therefore, harmonic components generated due to nonlinearity of the amplification means are suppressed by the filtering means together with the modulation product with frequency of xcex94f, also enhancing the SN ratio.
The above-described objects are achieved by a high-frequency amplifier comprising: amplification means for amplifying the multicarrier signals generated by combining plural carrier waves modulated independently and the filtering means connected to the output terminal of said amplification means, having a passband lying within the range of the band occupied by said multicarrier signal and a rejection band having a band in which second harmonic of said multicarrier signal is distributed or a higher rejection band containing the said band allotted to it.
As long as filtering characteristics adaptive to the characteristics of the amplification means and desired frequency allocation are established into the filtering means beforehand, the small-scale circuit including the amplification means and the filtering means described above can accomplish high power efficiency and high SN ratio in this high-frequency amplifier.
The objects described above can also be accomplished by connecting the filtering means in parallel between said amplification means 11 and the load connected to an output side of said amplification means 11.
In the high-frequency amplifier configured as described above, the input/output impedance of the filtering means is given a value suitable for the output impedance of the amplification means and for the impedance of the load, as long as these impedance have known values.
Accordingly, the filtering characteristics of the filtering means can be set at a desired accuracy as a transfer function suitable for such impedance.
Furthermore, the objects described above are achieved by amplification element that actively accomplishes the amplification function of the amplification means configuring the high-frequency amplifier described above and which is integrated with the filtering means forming the high-frequency amplifier, thus forming an integrated circuit.
In the amplification element configured in this way, the length of a line used for the filtering means to terminate the output terminal of the amplification means is shortened by the integration with the amplification elements as described above.
Accordingly, the impedance of this line is smaller compared with the case in which the filtering means are placed outside.
High SN ratio can be achieved even if there are a large number of carrier waves used for the generation of multicarrier signals to be amplified or even if the frequency differences in the frequency axis of these carrier waves are large.
Other objects and features of the invention will be clearly shown in the following explanation based on the accompanying drawings.